Online community building has been something I’ve been doing largely intuitively, and with a lot of experimentation and openness to try out different ways of engaging and onboarding people, for almost 7 years now. Trust in Play has a strong focus and purpose, and so it was somewhat less of a challenge to make it work. This would be my first tip: if you build a community, especially one from the scratch, think of the role it will play for its participants. Otherwise, you might end up doing exploitative work - when your main objective is to collect knowledge, ensure that this knowledge is benefiting those who participate, if not a wider community around these nodes.
In order to build a critical mass of people, think of an attractive call. We had an easy mission here - there was a wonderful training opportunity, and instead of going for a traditional application process via email, we channeled applicants here. I am not sure we did it 100% right, and I think one rather obvious thing to keep in mind is to write your questions in a way that they do not compromise too much of your members’ identity, especially when it comes to platforms openly available to wider audiences, and ask for things they’d be probably comfortable sharing in this context. No sensitive data. Hidden personal information. Nicknames can be as weird as they get. Be mindful of the current debates around privacy online, listen to tips from those who are more versed in the topic, and ideally encourage changes towards more privacy on the forum.
Once you have content pouring in, and members signing up, you need to keep the pace and read what they write. Respond. Make connections. Bring people together by pointing out who seems to have interests in common. This is the core of community management - being informed about your members, and trying to become as familiar and close to them as the medium allows you to, and becoming the connective tissue between people. You’re the hyper connector here. That does not mean reading every single thread, as it soon becomes unhealthy, but having a good overview of what’s going on where.
I like to be generous, so whenever I can, I look for opportunities for them to grow, meet, get support, learn. This is why we used a mix of webinars - which had a very steady following - that brought experts from inside and outside the community who shared their knowledge on topics that seemed to feed the interest of the groups pretty well. I might have misjudged a thing or two, and I’d be very happy to receive some criticism and feedback on that, but my intention was always to have a relatively good idea of what seems to be needed, and then to build around it. I tried different ways, and maybe my favorite was just to let two people who specialize in consent and safety, talk with each other. Never forget about questions from the public!
I would say that the golden rule for a person that runs a community should probably be generosity: bring more than you take out of it, at least to a point when members start being so generous themselves, you couldn’t possibly outdo them 
And do not forget to pay everyone who does their work, whenever you can. If a project has a budget, and your organization somehow benefits financially from the fact that this community exists and shares, invest back into it.
Offline events are often a luxury for widely dispersed groups, yet very much recommended. There’s nothing that feels like bringing together a bunch of people who met online in one place and organise an event for them. It’s really one of the most beautiful and fun things one can do as a community manager.
Fun online events are also recommended - I really encourage everyone to follow whatever artsy zooms and conferences in Minecraft they can find, in order to get an idea of the tools and hacks people use to make sitting in front of a screen fun. We did our Solstice Party to celebrate the school, and it was a huge achievement - 9 hours marathon of weird sessions, dancing, cooking, walking. Remember to keep the energy high and design into it breaks and sessions that happen far from the screen, yet in connection, for example via telegram. It feels good.
I’ve also added a bit of smaller ideas to the mix - I tried a grant writing lab, where I connected and supported some community members in their work, but also in establishing possible new partnerships. And I offered myself for consultations - without claiming to be an expert, more of a peer to peer clinic, where people could book an hour-long call with me and discuss their project idea further, and then I’d be suggesting some ways of bringing money and support into it. That went surprisingly well once or twice 
One last thing, since this post is already incredibly long, would be to show that you are available and open, and you’re there to talk about things if people need it. And do not worry you might not have the right answers - you might as well have something in mind that is very relevant, even if you don’t think it is. Being vulnerable and open is great, it’s probably better than being really smart Since you’re there to make people connect, stay curious, and talk to each other, I’d recommend a very humble attitude.
I guess since our mission is over, the challenge ahead of this community is to stay relevant beyond the project - we shall see how we manage. Nevertheless, I also see that the community lives, since I’ve been meeting some of you around Berlin in the past months, and I noticed TiP was a point of reference when we introduced each other to new people. I think it’s a good sign
yours,
nat